use fadvise to control Linux page-cache (#13312)

This PR brings two optimizations mainly
for page-cache build-up and how to avoid
getting OOM killed in the process. Although
these memories are reclaimable Linux is not
fast enough to reclaim them as needed on a
very busy system. fadvise is a system call
implemented in Linux to advise page-cache to
avoid overload as we get significant amount
of requests on the server.

- FADV_SEQUENTIAL tells that all I/O from now
  is going to be sequential, allowing for more
  resposive throughput.

- FADV_NOREUSE tells kernel to start removing
  things for this 'fd' from page-cache.

internal/disk/fdatasync_unix.go

@@ -29,3 +29,15 @@ import (
 func Fdatasync(f *os.File) error {
 	return syscall.Fsync(int(f.Fd()))
 }
+
+// fdavise advice constants
+const (
+	FadvSequential = 0
+	FadvNoReuse    = 0
+)
+
+// Fadvise implements possibility of choosing
+// offset: 0, length: 0
+func Fadvise(f *os.File, advice int) error {
+	return nil
+}